Researchers reviewed 22 studies covering 680 people, aged 19 to 72, who took protein supplements during weight training. Before training, participants were consuming an average of about a half-gram of protein per pound of body weight per day. For the study, doctors added 42 grams of whey, casein, and/or milk proteins per day, or a placebo.

After six weeks of weight training, compared to placebo, those who took protein added 1.5 pounds more fat-free mass, with those younger than age 50 adding 1.8 pounds of fat-free mass. Also for the protein group, both types of muscle fibers—those for running marathons, and those for sprints—increased by about 50 percent, and the maximum single-repetition lift outperformed placebo by 30 pounds.

Doctors concluded protein supplements are an effective way to improve resistance-type exercise and support healthy aging.

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